变压器原理介绍

Flat Transformers for Low Voltage, High Current,

High Frequency Power Converters

D. Trevor Holmes & K. Kit Sum

Flat Transformer Technology Corporation

3122 Alpine Avenue

Santa Ana, California 92704, U.S.A.

Tel.: (714) 708 7090 +++ Fax: (714) 708 7091

Abstract: The flat transformer is a magnetic structure comprising a number of

elements, N e, each of which can be identified as an individual transformer by itself.

These elements are arranged to obtain a transformation ratio (an equivalent turns

ratio) of 1 :1N

, or N e: 1, with a single turn secondary winding. With a primary

e

having a number of turns N p, the transformation ratio is N p× N e: 1.

The flat transformer is particularly well asuited to low profile, low output voltage, high current applications for high frequency switched-mode power

conversion applications. It has extremely low leakage inductance, high magnetization

inductance, excellent coupling, very low temperature rise, and is easily insulated for

high dielectric requirements with no appreciable degradation in performance.

Now commercially available, the FLAT TRANSFORMER AND INDUCTOR MODULES have very low profile. These modules can exceed most high density

high power converter performance expectations.

Fundamental Concepts

In a low output voltage high frequency conventional transformer, the output winding is usually configured to have a single turn. If the transformer has a turns ratio of n to 1, then the primary winding will have n turns. The number of primary turns cannot be less than n, by design.

In the flat transformer [1], the magnetic structure is comprised of many ferrite cores. The ferrite cores can be arranged in groups or elements of 1 to n in number. That is to say, 1 or more ferrite cores can be a “group” or “element” by itself; and the complete transformer is built with more than one of these “elements.”

Turns ratios are usually referred to a single core. When a group of cores are used, the resulting equivalent turns ratio is not reflected in individual cores, but rather is a reflection of the transformation ratio of the whole. Based on this principle, a single primary turn is feasible with the flat transformer.

+V

V

3

Figure 1. Flat Transformer with Transformation Ratio of 3 : 1

In the most basic arrangement of the flat transformer, a single turn is used for the primary as well as the secondary; and the transformation ratio of this flat transformer is now determined by the number of elements, N e.

Higher transformation ratios may be obtained by using more than one primary turn. The transformation ratio of the flat transformer is determined by the product of the number of elements, N e, and the number of primary turns, N p:

Transformation Ratio = (N e× N p) to 1 (N p=2, shown below)

V

—

6

Figure 2. Flat Transformer with 3 Elements and Transformation Ratio of 6 : 1

Flat Transformer-Inductor Module Design

This module is optimized for bridge or half-bridge converters providing a 5-volt output with a single secondary turn, and operates at a switching frequency of 250 kHz (500 kHz output ripple). Each module can have a power delivery capacity of 150 watts.

The transformer part of this module is made up of two rectangular ferrite blocks with a square hole at the center. See Figure 3(a). An additional block, with a total of three blocks, is added to the combination to perform the function of the inductor.

Two single-turn secondary windings are inserted in each block to incorporate the output winding with a center-tap. Each turn is bonded to the inside surface of the block, and follows a 180°helical path such that the turn connects from one outside corner diagonally to the other outside corner. See Figure 3(b). The two blocks are arranged to have the secondary winding of one block connected in series with the adjacent block. The connections to the secondary windings are located at the corners of the blocks as shown in Figure 3(c).

(a)(b) (c)

Figure 3. (a) Single Core Block, (b) Core Block with Helical Windings,

and (c) Two Blocks Connected Together.

This secondary winding design succeeds in eliminating the labor normally required of fabricating a tapped secondary winding.